It is known to equip security documents not only with a printed image applied by steel intaglio printing but also with special authenticity features, those of interest for the present invention being in particular optically variable elements such as embossed holograms or grids (DE-A-40 02 979) and blind embossings (DE-A-198 45 552).
Blind embossings are occasionally produced together with the steel intaglio printed image in a common printing operation using one partially inked steel intaglio printing plate. During the printing operation the paper is pressed into the depressions of the blind embossing areas and thus lastingly deformed. The blind embossing areas of the printing plate are not filled with ink, unlike the printed image areas, so that the substrate material of the security document is only lastingly deformed, i.e. embossed, in these areas (WO 97/48555; DE-A-198 45 552).
When blind embossings are viewed, light and shadow effects produce special three-dimensional optical impressions. In addition, blind embossings with suitable dimensions can also be easily detected tactilely.
The structures for the steel intaglio printed image and for the blind embossings are usually incorporated in the printing plate surface by means of a graver, laser or by etching. Regardless of the incorporation technology used, these structures will also be referred to in general as “engravings” in the following. The fineness of the structures is limited, however, firstly by the employed engraving techniques themselves, but secondly by the fact that especially fine structures do not long withstand the mechanical influences of the wiping cylinder used for wiping surplus ink off the partially inked printing plate. The lightly reciprocating motion and the friction prevailing at a corresponding contact pressure of the wiping cylinder cause embossed structures of an order of magnitude of distinctly less than 100 microns (referred to as “microstructures” in the following) to be damaged within a very short time. Embossings with microstructures distinctly smaller than 100 microns for producing special optical effects are accordingly produced in an embossing operation performed separately from the printing operation for applying the steel intaglio printed image.
The same holds for the application of optical diffraction structures such as holograms and grids. The order of magnitude of these diffraction structures is within the wavelength range of visible light, i.e. under 1 micron. In DE-A-198 45 552 it is proposed that a paper of value be prefabricated with all security elements, including for example embossed diffraction structures, and the paper printed as the last method step for example by steel intaglio printing. It is described in this context as a possible variant that the diffraction structures are built up in layers on a previously locally smoothed area of the paper-of-value substrate by first applying a curable lacquer to the smoothed area and providing it with an extremely thin, reflective metal layer. A diffractive relief structure is then embossed into this coated lacquer layer with an embossing die, and the thus produced diffraction structure then covered with a protective lacquer.
Producing embossed microstructures in a security document, whether as a blind embossing in the substrate material itself or as a diffractive relief structure in a specially provided lacquer layer, thus requires a separate working step in addition to the printing operation for producing the steel intaglio printed image.